Not sure it matters which meaning OP wants. You've accurately summarised the distinction between including the article or not, so he can just choose the appropriate form without further assistance. Good short answer, btw.
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FumbleFingersJul 7 '11 at 16:03

I meant that it's important, so I guess my initial wording was right. Thanks!
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LaCJul 7 '11 at 16:05

BTW, "is the key" sounds closer to "the key solution" to me, while I just want to indicate an important aspect of the problem. Is my feeling right?
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LaCJul 7 '11 at 16:44

1

I prefer just is key unless you are arguing that there is nothing else to consider or must always be considered above all else. Inserting the word the here infers that anyone who thinks something else is equally or more important is wrong. As I can probably find a situation in a specific distributed application where concurency is not the most important factor I would stick with just "IS KEY".
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ChadJul 7 '11 at 17:16